Japanese patent Kokai publication No. Hei 5-290793 discloses a photomultiplier tube in which an electron multiplier is accommodated in a hermetically sealed vessel. The vessel has a metal side tube having a flange at an upper end. The flange is welded and fixed to an upper surface of a faceplate, thereby ensuring airtightness of the vessel. The flange of the side tube is welded to the faceplate, while the side tube is heated.
However, the following problem arises as to a conventional photomultiplier tube. Referring to FIG. 18, a side tube 100 has a flange 101 provided at the entire upper end thereof. A lower face 101a of the flange 101 is in contact with an upper face 102a of a faceplate 102, so that the side tube 100 and the faceplate 102 are fused. Such a photomultiplier tube has a flange 101 overhanging the upper face 102a of the faceplate 102. The flange 101 covers an edge of the faceplate 102 at an upper end of the side tube 100. The flange 101 narrows the upper face 102a of the faceplate 102, thereby decreasing an effective sensitive area of the faceplate 102. Recently, many photomultiplier tubes are frequently arranged in a single detector for a certain application. In this case, it is desired to increase an effective sensitive area of the faceplate 102 even by 1%. The dense arrangement of the photomultiplier tubes in the detector, however, may generate a significant amount of dead space in the detector. Therefore, it is difficult to improve performances of the detector due to the above problem.
In view of the foregoing, it is an object of the present invention to provide a photomultiplier tube having an increased effective sensitive area of the faceplate and an increased fix area of the side tube to the faceplate.
It is another object of the present invention to provide a photomultiplier tube unit having an increased effective sensitive area of the faceplate.
It is further object of the present invention to provide a photomultiplier tube unit facilitating a gain control (current gain) of each electron multiplier in the side tube.
It is still further object to provide a radiation detector having improved performances over the entire detector based on the enlarged effective sensitive area of the faceplate.